Thymic stromal lymphopoietin (TSLP) is a growth factor integral to both B and T cell development and maturation. In particular, murine TSLP supports B lymphopoieses and is required for B cell proliferation. Murine TSLP is also critical in controlling the rearrangement of the T cell receptor-gamma (TCRγ) locus, and has a substantial stimulatory effect on thymocytes and mature T cells. See, for example, Friend et al., 1994, Exp. Hematol., 22:321-328; Ray et al., 1996, Eur. J. Immunol., 26:10-16; Candeias et al., 1997, Immunology Letters, 57:9-14.
TSLP possess cytokine activity similar to IL-7. For example, TSLP can replace IL-7 in stimulating B cell proliferation responses (Friend et al., supra). TSLP and IL-7 appear to mediate their lymphopoietic effects via distinct mechanisms. For example, IL-7 activates Janus family tyrosine kinases, including JAK1 and JAK3, and modulates the activity of the signal transducer and activator of transcription 5 (STAT5) protein. While TSLP modulates the activity of STAT5, it fails to activate any Janus family tyrosine kinase members (Levin et. al., 1999, J. Immunol. 162:677-683). Although TSLP and IL-7 mediate similar effects on target cells, they also appear to have distinct signaling pathways and likely some variation in their biologic response.
The known activities of TSLP in modulating the immune system, particularly in stimulating B and T cell proliferation, development, and maturation, makes this molecule an attractive therapeutic tool. The ability to produce large quantities of the active polypeptide is essential to commercial production of human TSLP. Production of recombinant polypeptides in a mammalian cell expression system is most commonly used for commercial human therapeutic applications.
Recombinant huTSLP polypeptide has been expressed in a number of different expression systems, including mammalian cell lines, as described in WO 00/29581. However, production of useful quantities of active huTSLP protein in mammalian cells has been difficult. In particular, huTSLP expression in mammalian cells often yields a degraded product. Alternative polynucleotide molecules and methods to achieve production of useful quantities of active huTSLP polypeptide are needed.